1. Field of the Invention
The present invention relates to a method of preparing an electrode assembly, in which both sides of a single current collector are coated to form an anode and a cathode, and the current collector is then bent into a vertical sectional zigzag shape and integrated in a state of disposing a separator at interfaces between facing electrode patterns, and an electrode assembly prepared by the method.
2. Description of the Related Art
Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage, long cycle lifetime, and low self-discharge rate have been commercialized and widely used.
Lithium secondary batteries are classified as a lithium-ion battery, a lithium-ion polymer battery, and a lithium polymer battery according to the configurations of an electrode assembly having a structure of cathode/separator/anode and an electrolyte solution.
Also, the lithium secondary batteries are broadly categorized as a jelly-roll type (winding-type) and a stack-type (laminate-type) according to the structure of the electrode assembly. For example, a jelly-roll type electrode assembly is prepared in such a manner that a metal foil used as a current collector is coated with an electrode active material, dried, and pressed to prepare an electrode, the electrode is cut in the shape of a band having desired width and length, and the jelly-roll type electrode assembly is then prepared by separating a cathode and an anode using a separator and winding the resultant product in a spiral shape. The jelly-roll type electrode assembly is appropriate for a cylindrical type battery. However, the jelly-roll type electrode assembly may have disadvantages, such as the exfoliation of the electrode active material and low space utilization, when used in a prismatic type or pouch type battery. The stack-type electrode assembly has a structure in which a plurality of cathode and anode units are sequentially stacked, wherein the stack-type electrode assembly may have advantages in that a prismatic shape may be easily obtained, but may have disadvantages in that a preparation process is cumbersome and an electrode is pushed to cause a short circuit when an impact is applied thereto.
In order to address the above limitations, an electrode assembly having a structure, in which a predetermined sized full cell composed of cathode/separator/anode or a predetermined sized bi-cell composed of cathode (anode)/separator/anode (cathode)/separator/cathode (anode) is sequentially stacked to allow the cathode and the anode to face each other in the state of disposing a long continuous separation film therebetween, was developed as described in Korean Patent Application Laid-Open Publication Nos. 2001-82058, 2001-82059, and 2001-82060.
A structure of such a layered electrode assembly may be identified by schematic views of preparing processes illustrated in FIGS. 1A and 1B.
Referring to FIGS. 1A and 1B, since a layered electrode assembly 10 is prepared through various processes, such as a process of cutting a cathode 1, an anode 2, and a separator 5 to a predetermined size, a process of preparing a bi-cell 6 (or full cell) by sequentially stacking the cut cathode 1, anode 2, and separator 5 (see FIG. 1A), a process of folding the bi-cell 6 thus prepared by using a separation film 7 (see FIG. 1B), and a process of electrically connecting electrode tabs 3 and 4 protruding from one sides of the cathode 1 and the anode 2, a preparation process is cumbersome and preparation costs and time according to the number of processes may be increased. Also, in the process of connecting the electrode tabs 3 and 4, since a separate connecting member is not only essential but difficult work, such as welding, is also performed, this may be a cause of increasing the probability of generating defects of a battery as well as manufacturing costs.
Recently, in order to improve such limitations, a method of preparing a vertical sectional zigzag-shaped electrode assembly, instead of an electrode assembly having stack-type and layered stack structures, is known.
For example, Korean Patent No. 10-0907623 discloses a method of preparing an electrode assembly for a secondary battery in which an electrode having one side of a current collector coated with an active material layer is bent in a vertical sectional zigzag shape and the electrode is fit to allow the active material layers to face each other in a state of disposing a separator therebetween.
However, similar to a typical layered electrode assembly, since the method requires a process of preparing electrode units by respectively coating the electrode current collectors with cathode and anode active materials and aligning the separator therebetween, the method has fundamental limitations.
Also, Korean Patent No. 10-0303119 discloses a method of preparing an electrode assembly in which electrode sheet and separator are prepared in a stack structure of cathode/separator/anode/separator and the stack structure is then folded in a zigzag shape.
However, in this method, since each electrode is also formed on each electrode current collector and an electrode assembly is then formed by bending the stack structure of the electrodes and separators, a process of bending is not facilitated due to a predetermined thickness of each electrode and separator, and also, the electrode active material layer on the current collector may be exfoliated in the process of applying force to perform the bending process.
Thus, an electrode assembly prepared by a typical stack, stack and folding, or folding method may have limitations in that an amount of a current collector used for preparing an electrode is large or an entire structure is distorted during the preparation or expansion of a battery. In particular, since a cathode and an anode are each prepared in a separate two-step process despite the fact that current collector materials for preparing the cathode and the anode are the same, preparation costs and time may increase.
Therefore, there is a need to develop a new technique for preparing an electrode assembly which may fundamentally resolve the above limitations and may simultaneously reduce preparation costs and time.